In a synchronous transmission system or network, data traffic takes the form of synchronous data signals normally comprised of data frames. Data traffic is normally transported in high order or low order data structures, where a high order structure may contain more than one low order structure. For example, in an SDH (Synchronous Digital Hierarchy) system, data is normally carried in structures known as Virtual Containers (VCs) which may be high order, for example a VC-4, or low order, for example a VC-11 or VC-12. A VC-4 may comprise, for example, 63 VC-12s or, say, 2 low order VC-3s and 21 VC-12s. The Virtual Containers are transported across a network in frames known as STM (Synchronous Transport Module) frames where, for example an STM-1 frame transports a VC-4 at a transmission rate of 155.52 Mbit/s. Similarly, in a SONET (Synchronous Optical Network) system, data may be carried in high order structures known as STSs (Synchronous Transport Signals) or low order structures known as VTs (Virtual Tributaries). By way of example, a SONET STS-3 (155.52 Mbit/s) is comparable with an SDH STM-1 and may comprise 3 STS-1s (51.84 Mbit/s). An STS-1 may comprise 21 VT2s, where VT2s are comparable with SDH VC-12s.
In SDH networks, it is necessary for a switching element to be able to control data payloads at the high order level (e.g. at STS or VC-4 level) and desirable for it to be able to control payloads at the low order level (e.g. at VT or VC-12 level). For convenience, SONET terminology will be used primarily hereinafter. For example, high order switching is referred to as STS switching, and a high order switch is referred to as an STS switch, while low order switching is referred to as VT switching and a low order switch is referred to as a VT switch. Similarly, a low order alignment apparatus is referred to as a VT aligner. It will be understood however that the invention applies equally to equivalent SDH, and other, structures and apparatus (e.g. TU switching and TU alignment).
In previous generations of products (STM-1/STM-4 in ETSI), it has been normal for equipment to support full VT switching i.e. to be able to manage payloads down to the VC-12 (2 Mbit/s), or equivalent, level. However, with the rapid increase in the transmission rates used in SDH/SONET networks (to STM-64 and STM-256) it is no longer desirable, or in some cases practical, for network equipment to provide full VT switching. It is, however, still desirable for network equipment to be able to provide at least some VT switching. This is commonly known as partial VT switching.
One approach for providing partial VT switching is to subtend a VT switching apparatus from an STS switching apparatus. In such an arrangement, the STS switch passes to the VT switch only traffic comprising VT traffic that requires to be switched (where switching normally takes the form of timeslot interchanging and/or port interchanging) all other traffic being handled by the STS switch alone. This allows a less complicated version of the VT switching apparatus to be used than is needed for full VT switching. This is desirable since, in general, VT switching apparatus are relatively complex, expensive and operate at slower rates than STS switches.
A problem with subtending a VT switch in this way is that VT alignment is conventionally required between the STS switching apparatus and the VT switching apparatus in order to compensate for delays incurred in passing the traffic through the STS switching apparatus and the VT switching apparatus before it is routed back to the STS switching apparatus. A VT alignment apparatus is relatively expensive and adds significantly to the overall cost of the switching apparatus.